Strong oxidants, including hydrogen peroxide, peracetic acid, and chlorine dioxide, are effective antimicrobials, including efficacy against bacterial and fungal plant pests. When the pesticide's mode of action is oxidation, for example oxidation of the target organism's cell wall, the target organisms are not likely to develop resistance to the pesticide. Oxidants such as hydrogen peroxide, peracetic acid, and chlorine dioxide readily decompose in the environment to harmless products. A pesticide should preferably have a long residence time; that is, it should remain effective after application to the plant for several days or longer, to minimize the cost and labor of re-application.
There are commercial pesticide products using hydrogen peroxide and peracetic acid. However, their utility is limited by their relatively short residence time on the plant surface. Chlorine dioxide could potentially be an active component in a plant pesticide formulation, however limitations have prevented its use. A solution of chlorine dioxide in water may be generated by combining water solutions of sodium chlorite and an acid or transition metal. The acid may be a mineral acid, such as hydrochloric acid, or an organic (carboxylic) acid, including hydroxycarboxylic acids such as ascorbic acid or citric acid.
One drawback that has prevented the use of chlorine dioxide as a plant pesticide is the fact that it readily decomposes in the environment, limiting the residence time on the plant. In addition, chlorine dioxide is a gas at room temperature; but it is very soluble in water, and is commonly used in water solution. However, when a water solution of chlorine dioxide is sprayed, for example to apply the solution to a plant surface, the chlorine dioxide can disperse (vaporize) into the atmosphere. This is undesirable for two reasons: first, loss of chlorine dioxide decreases the antimicrobial activity; second, chlorine dioxide in the atmosphere produces an inhalation hazard. The permissible exposure level (PEL) for human workers for chlorine dioxide in air is 0.1 ppm. Chlorine dioxide in the atmosphere is hazardous and may require workers to wear Personal Protective Equipment (PPE).
Plants require protection from harmful organisms (pests) including pathogens (that result in plant diseases), bacteria, fungi, viruses, mollusks, worms, arthropods, and arthropod eggs. Existing plant protection products can mitigate the damaging influences of plant diseases and insects using various mechanisms. It is desirable for plant protection products to mitigate a threat, but also to minimize damage to the plant, to minimize the development of resistant microorganisms, to minimize toxicity to humans and other species, and to not harm the environment.
Current practice recognizes the efficacy of activated halogen species as a disinfectant to protect plants against pathogens. For example, Bliss and Bliss (2015; U.S. Pat. No. 9,018,239) describe addition of an anti-pathogen composition based on halogens to irrigation water. Similarly, Yao et al. (2010) show that ClO2 in irrigation water was effective in protecting calla lilies and other flowers against bacteria and fungi.
Yet another attempt at eliminating microorganisms on surface and in biofilms is through the use of a water-insoluble photoactivator to produce chlorine dioxide. Specifically, it is known to use titanium dioxide (TiO2) and a chlorine dioxide precursor in conjunction with exposure to ultraviolet light to generate chlorine dioxide. However, such processes are undesirable due to the use of an insoluble inorganic photoactivator. In addition, titanium dioxide forms particulates which leave undesirable residue. Averett and Averett (2015; U.S. Pat. No. 9,055,751) describe application of a photocatalytic composition comprising zinc-doped titanium dioxide nanoparticles to the surface of plants to treat or prevent plant diseases.
Riggs et al. (2014, U.S. Pat. No. 8,748,347) describe application of materials to plant surfaces to filter out UV light, for the purpose of protecting plants against harmful fungi and bacteria. More specifically, Riggs et al. describe “methods and compositions for controlling, preventing, or treating plant pathogens using UV filters for combating phytotoxin-producing fungi and/or bacteria.”
US 2015/0210964, published Jul. 30, 2015, teaches product compositions that include one or more photoactivators to generate one or more benefit active agents, effective as a bleaching agent, stain remover, or antimicrobial and/or in eliminating biofilm. 2015/0210964 also relates to methods for cleaning and/or bleaching surfaces, and for providing a method of disinfecting or sanitizing surfaces and/or removing biofilm. 2015/0210964 is incorporated by reference herein.
To control diseases, growers predominantly use chemical fungicides (Coyier and Roane, 1986; Chase et al., 2005). In 2009, 870 thousand pounds of fungicides (22.4% of the total pesticides) were applied to floriculture and nursery crops (USDA, 2011). Potential adverse impacts of chemicals on the environment and emergence of fungicide-resistant pathogen strains have put pesticides under increased public scrutiny.
These above references contain at least one of the following limitations in regard to protecting plants from harmful organisms: inability to kill or mitigate the harmful organism or its negative effect on the plant, inability to avoid the development of resistant organisms, inability to use non-toxic or environmentally friendly compounds, inability to generate a suitable benefit species within a controlled concentration range for an extended period of time, inability to generate the benefit active species in situ and minimize volatilization, and inability to treat the pest without degrading the commercial value of the plant.